1. Field of the Invention
This invention relates to a method of manufacturing a building product by mixing a hydraulic binding agent, preferably Portland cement, as a matrix with a fibre reinforcement and to the product so manufactured. Particularly, but not exclusively, the invention is concerned with the manufacture of fibre reinforced building sheets and the composition of sheets so manufactured.
2. Description of the Prior Art
Fibre reinforced building products of the construction described are well known. Such products are based on a matrix consisting mainly of a hydraulic binding agent such as Portland cement, alumina cement, fly ash cement, lime, gypsum, diatomaceous earth, puzzolan and mixtures thereof. Fibres used for the reinforcement may be organic or inorganic fibres or a mixture of fibres, and fibres which have been proposed include asbestos, glass fibres, steel fibres, mineral fibres, cellulose fibres and plastics fibres.
The resources of asbestos fibres suitable for the production of such fibre reinforced building products are limited and for many purposes it is desirable to avoid the use of asbestos as reinforcement fibres. Further it is a drawback when using glass fibres and steel fibres that such fibres are very expensive, and to achieve the same extent of reinforcement the expenditure will be 4-5 times as high as if asbestos had been used. Besides, glass fibres and steel fibres are inclined to decompose to some extent, the latter only insignificantly though, and only at the surface of the fibre reinforced material, whereas the former continuously undergo a certain decomposition as a consequence of the alkali-nature of the cement.
From British Pat. Specification No. 1,130,612 it is known to use fibrous reinforcement components made from an elongated and subsequently fibrillated plastic film material, preferably a polyolefin film. The abovementioned disadvantages in connection with asbestos fibres, glass fibres and steel fibres do not occur when the aforesaid material is used in a mortar consisting of e.g. Portland cement and gravel, but because of their extremely smooth surface only a poor bondage to the matrix after setting of same is achieved. The fibre reinforcement known from said British patent specification consists of twisted plastic twine weighing 1-1.5 g/m, and having been cut to filaments of approx. 75 mm. The twine is made as a so-called split fibre material from a film-shaped plastic band, which upon extrusion and cooling has been stretched approx. ten times its original length. The stretching causes the structure of the plastic material to become more orientated with resultant significantly increased tensile strength lengthwise, and significantly reduced tensile strength perpendicularly thereto.
Because of its poor tensile strength crosswise, the elongated material gets a natural tendency to split-up; this splitting-up may e.g. be brought about by twisting the stretched plastic film band around its longitudinal axis, and by further mechanical impact, e.g. as a consequence of blows or forces of friction from stone and gravel particles during admixing in a concrete mixer, in a method according to which they are split-up in several thinner strings. By the known splitting-up the division into thinner strings will always take place along the native weakest lines or areas of the material parallel to the direction of elongation of the band. Thus the fibres formed get rectangular cross-sections, always completely smooth surfaces, and constant cross-section throughout their length, this being the reason why it has been impossible to achieve an anchorage more satisfying than is the case with the usual round fibres. The smooth surface and the constant cross-section of the plastic fibres used up to now also account for the inadequate control of a homogeneous crack distribution in the hardened plastic fibre reinforced product. This disadvantage is further aggravated by the fact that the crosswise contraction properties of the plastic materials are extraordinarily high, viz. 0.4-0.5 as compared with approx 0.2 in case of glass and approx. 0.3 in case of steel. Thus by elongation, the smooth fibre will, as soon as strain is applied, get thinner, by which it loses intimate contact with the surrounding matrix along smaller or larger areas of its circumference, and along its entire longitudinal direction.
As a remedy it has been suggested only to let the splitting-up take place along a section of the full length of the twine, offset within the fibre bundle, so that when spreading the bundle, a material is achieved which may resemble netting having various widths of meshes. It is said that by this is achieved an anchorage which cannot be obtained with the usual round plastic fibres, but also the reinforcement of this kind is insufficient. The comparatively few crossing points, which by the way separately are rather weak as the splitting-up may just continue, are unable to a significant extent to surmount the influence of the very long areas with completely smooth cleavage faces, deriving from the splitting-up, to which a cement matrix cannot adhere, and consequently they do not yield the desired effect. The known plastic fibre twine is shown in FIG. 1 on the drawing.
The known splitting up of fibres may take place by exposing the stretched plastic film to a certain physical treatment. However, they form coherent bundles of fibre filaments even after the vigorous handling they are exposed to as a result of the mixing with gravel and stone in the concrete mixer. The cross-section of the individual fibres will still be so large that a certain resilient movement will occur when the fibres have been bent, which is the case after compression of the fibre containing concrete mass, and this in itself causes a poor anchorage of the fibres too.